Large amounts of carbohydrates in plant biomass provide a plentiful source of potential energy in the form of sugars (both five carbon and six carbon sugars) that can be utilized for numerous industrial and agricultural processes. However, the enormous energy potential of these carbohydrates is currently under-utilized because the sugars are locked in complex polymers, and hence are not readily accessible for fermentation. These complex polymers are often referred to collectively as lignocellulose. Sugars generated from degradation of plant biomass potentially represent plentiful, economically competitive feedstocks for fermentation into chemicals, plastics, and fuels, including ethanol as a substitute for petroleum.
For example, distillers' dried grains (DDG) are lignocellulosic byproducts of the corn dry milling process. Milled whole corn kernels are treated with amylases to liquefy the starch within the kernels and hydrolyze it to glucose. The glucose so produced is then fermented in a second step to ethanol. The residual solids after the ethanol fermentation and distillation are centrifuged and dried, and the resulting product is DDG, which is used as an animal feed stock. Although DDG composition can vary, a typical composition for DDG is: 32% hemicellulose, 22% cellulose, 30% protein, 10% lipids, 4% residual starch, and 4% inorganics. In theory, the cellulose and hemicellulose fractions, comprising about 54% of the weight of the DDG, can be efficiently hydrolyzed to fermentable sugars by enzymes; however, it has been found that the carbohydrates comprising lignocellulosic materials in DDG are more difficult to digest. To date, the efficiency of hydrolysis of these (hemi) cellulosic polymers by enzymes is much lower than the hydrolytic efficiency of starch, due to the more complex and recalcitrant nature of these substrates. Accordingly, the cost of producing the requisite enzymes is higher than the cost of producing amylases for starch hydrolysis.
Major polysaccharides comprising lignocellulosic materials include cellulose and hemicelluloses. The enzymatic hydrolysis of these polysaccharides to soluble sugars (and finally to monomers such as glucose, xylose and other hexoses and pentoses) is catalyzed by several enzymes acting in concert. For example, endo-1,4-β-glucanases (EGs) and exo-cellobiohydrolases (CBHs) catalyze the hydrolysis of insoluble cellulose to cellooligosachharides (with cellobiose the main product), while β-glucosidaes (BGLs) convert the oligosaccharides to glucose. Similarly, xylanases, together with other enzymes such as α-L-arabinofuranosidases, feruloyl and acetylxylan esterases and β-xylosidases, catalyze the hydrolysis of hemicelluloses.
Regardless of the type of cellulosic feedstock, the cost and hydrolytic efficiency of enzymes are major factors that restrict the widespread use of biomass bioconversion processes. The hydrolytic efficiency of a multi-enzyme complex in the process of lignocellulosic saccharification depends both on properties of the individual enzymes and the ratio of each enzyme within the complex.
Enzymes useful for the hydrolysis of complex polysaccharides are also highly useful in a variety of industrial textile applications, as well as industrial paper and pulp applications, and in the treatment of waste streams. For example, as an alternative to the use of pumice in the stone washing process, methods for treating cellulose-containing fabrics for clothing with hydrolytic enzymes, such as cellulases, are known to improve the softness or feel of such fabrics. Cellulases are also used in detergent compositions, either for the purpose of enhancing the cleaning ability of the composition or as a softening agent. Cellulases are also used in combination with polymeric agents in processes for providing a localized variation in the color density of fibers. Such enzymes can also be used for the saccharification of lignocellulosic biomass in waste streams, such as municipal solid waste, for biobleaching of wood pulp, and for deinking of recycled print paper. As with the hydrolysis of these polysaccharides in lignocellulosic materials for use as feedstocks described above, the cost and hydrolytic efficiency of the enzymes are major factors that control the use of enzymes in these processes.
Filamentous fungi are a source of cellulases and hemicellulases, as well as other enzymes useful in the enzymatic hydrolysis of major polysaccharides. In particular, strains of Trichoderma sp., such as T. viride, T. reesei and T. longibrachiatum, and Penicillium sp., and enzymes derived from these strains, have previously been used to hydrolyze crystalline cellulose. However, the costs associated with producing enzymes from these fungi, as well as the presence of additional, undesirable enzymes, remains a drawback. It is therefore desirable to produce inexpensive enzymes and enzyme mixtures that efficiently degrade cellulose and hemicellulose for use in a variety of agricultural and industrial applications.